The above mentioned parent applications disclose various techniques for embedding and detecting out-of-phase digital watermarks.
Digital watermarking technology, a form of steganography, encompasses a great variety of techniques by which plural bits of digital data are hidden in some other object, preferably without leaving human-apparent evidence of alteration.
Digital watermarking may be used to modify media content to embed a machine-readable code into the media content. The media may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process.
Digital watermarking systems typically have two primary components: an embedding component that embeds the watermark in the media content, and a reading component that detects and reads the embedded watermark. The embedding component embeds a watermark pattern by altering data samples of the media content. The reading component analyzes content to detect whether a watermark pattern is present. In applications where the watermark encodes information, the reading component extracts this information from the detected watermark. Assignee's U.S. patent application Ser. No. 09/503,881 (now U.S. Pat. No. 6,614,914), filed Feb. 14, 2000, discloses various encoding and decoding techniques. U.S. Pat. Nos. 5,862,260 and 6,122,403 disclose still others. Each of these U.S. patent documents is herein incorporated by reference.
Now consider our inventive out-of-phase digital watermarking techniques with reference to FIGS. 1a and 1b. In FIG. 1a, the dash/dot C, M, Y and K lines represent, respectively, cyan, magenta, yellow and black color channels for a line (or other area) of a media signal (e.g., a picture, image, media signal, document, etc.). The FIG. 1a lines represent a base level or a particular color (or gray-scale) level (or intensity). Of course, it is expected that the color (or gray-scale) level will vary over the media signal. FIG. 1b illustrates the media of FIG. 1a, which has been embedded with an out-of-phase digital watermark signal. The watermark signal is preferably applied to each of the color component dimensions C, M and Y.
In FIGS. 1a and 1b, the M and Y channels are represented by one signal, since these color components can be approximately equal, but separate signals. Of course, it is not necessary for these components to be equal, and in many cases the yellow and magenta components are not equal. The illustrated “bumps” (or “tweaks”) in FIG. 1b represent the digital watermark signal, e.g., upward and downward signal adjustments in relation to a respective color channel at given points over the media signal. The tweaks are preferably applied at the same level (or signal strength). Alternatively, the bumps are applied with a different signal strength (or tweak level) when compared to one another. Of course, these tweaks can be embedded over a color channel in a predetermined pattern, a pseudo random fashion, a random fashion, etc., to facilitate embedding of a digital watermark signal.
For the K dimension (or channel), the digital watermark signal is preferably embedded to be out-of-phase with respect to the CMY channels. Most preferably, the K channel is approximately 180 degrees out-of-phase (e.g., inverted) with the watermark signals in the CMY color channels, as shown in FIG. 1b. For example, if a digital watermark signal modifies each of the color channels at a media first location with a tweak level of say 7, then a tweak level of −7 correspondingly modifies the K channel at the media's first location. This inventive digital watermark technique is referred to as our out-of-phase (or “K-phase”) digital watermark. (We note that if a watermark signal is determined in terms of luminance, we can assign or weight corresponding tweak levels to the respective color plane pixel values to achieve the luminance value tweak. Indeed, a tweak can be spread over the CMY channels to achieve a collective luminance at a given media location. The luminance attributable to the CMY tweak is preferably cancelled or offset by the luminance effect attributable to a corresponding inverted K channel tweak at the give media location.).
Our inventive watermarking scheme greatly reduces watermark perceptibility. Since the watermark signal for the K channel is applied approximately 180 degrees out-of-phase, when compared to the respective tweaks applied to the C, M and/or Y channels, the watermark visibility is greatly reduced. The visibility reduction is achieved by the effective cancellation of perceived luminance changes when the CMYK image is viewed or printed. Indeed, combining an inverted watermark signal “tweak” or “bump” in a K channel with a corresponding non-inverted watermark signal tweak or bump in the CMY channels effectively cancels an overall perceived luminance change for a given area (e.g., a pixel or block of pixels)—greatly reducing visibility of the digital watermark.
The present invention discloses a new data hiding technique based on our out-of-phase technology. According to one implementation of the present invention, an image is hidden in or carried by a media signal. The hiding is accomplished with our out-of-phase embedding techniques. The image can be a photograph, a graphic, a barcode (1-D or 2-D), etc., etc. Another aspect of our inventive techniques is used to improve the visibility characteristics of our out-of-phase embedding techniques.
The foregoing and other aspects, features and advantages of the present invention will be even more apparent from the following detailed description, which proceeds with reference to the accompanying drawings.